Recently, energy saving operations have been required from a view point of environmental measures. In apparatuses using a battery such as a mobile phone and a digital camera, in order to make the battery life long, reduction of the power consumption in the apparatuses has been required.
In addition, an advanced functional apparatus has been developed. For example, instead of recording and reproducing a still image, recording and reproducing a moving image has been frequently performed in the advanced functional apparatus. Consequently, a high-end CPU has been used in the apparatus in which the clock frequency is high. However, when the clock frequency is high, the amount of current to be consumed becomes high. In addition, in order to make the clock frequency high, a high power source voltage is required. As a result, the power consumption becomes high.
In order to solve the above problem, technology for minimizing the power consumption in the apparatus has been developed. In the technology, at a normal operation time, the power source voltage is decreased, the CPU is operated in a low clock frequency, and the power consumption is made to be low; and at high operation time, for example, at a moving image operating time, the power source voltage is increased, and the CPU is operated in a high clock frequency.
In addition, a multifunctional apparatus has been developed, in which since power source voltages are different from each other in many functions in the apparatus, plural power sources having different output voltages are disposed in the apparatus. Further, when a high-speed operation is required, the plural output voltages from the plural power sources must be increased or decreased according to predetermined relationships among the plural output voltages.
As control technology of output voltages of plural power sources, Patent Document 1 discloses a method. In the method, information that changes an output voltage of a main power source is output to a sub power source, and the sub power source determines an output voltage based on the received information.
In Patent Document 2, an output voltage of a sub power source is determined to be proportional to an output voltage of a main power source, and when the output voltage of the main power source is changed, the output voltage of the sub power source is proportionally changed.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2003-304679
[Patent Document 2] Japanese Unexamined Patent Publication No. S58-224562
However, conventionally, only the output voltages are determined, and in the middle of the changes of the output voltages, no control of any kind has been disclosed.
FIG. 5A is a graph showing voltage change characteristics when an output voltage Vom of a main power source circuit and an output voltage Vo1 of a sub power source circuit are increased or decreased. In FIG. 5A, at a low-speed operation time of a load circuit, for example, the output voltage Vom of the main power source circuit is determined to be 0.7 V and the output voltage Vo1 of the sub power source circuit is determined to be 1.1 V. At a high-speed operation time of the load circuit, the output voltage Vom of the main power source and the output voltage Vo1 of the sub power source are increased to 1.4 V. When the operation is returned to the low-speed operation, the output voltage Vom of the main power source circuit is returned to 0.7 V and the output voltage Vo1 of the sub power source circuit is returned to 1.1 V.
In conditions when the output voltages are increased, first, the output voltage Vom (lower than the output voltage Vo1) of the main power source circuit is increased. When the output voltage Vom becomes approximately equal to the output voltage Vo1, the output voltage Vo1 is increased at an approximately equal changing speed to the changing speed of the output voltage Vom. At this time, a difference between the output voltages Vom and Vo1 must be within, for example, 50 mV. In addition, a difference between the output voltages Vom and Vo1 at the target voltage 1.4 V must be within, for example, 50 mV.
FIG. 5B is an enlarged view of a circled part of FIG. 5A. In FIG. 5B, a region sandwiched between alternate one-dot broken lines is the ±50 mV range from the output voltage Vom of the main power source circuit. When the output voltage Vom of the main power source circuit is increased and the difference between the output voltages Vom and Vo1 becomes within the range of ±50 mV, the output voltage Vo1 of the sub power source circuit is started to be increased and is increased at an approximately equal changing speed to the changing speed of the output voltage Vom of the main power source circuit. Therefore, the difference between the output voltages Vom and Vo1 is within the range of ±50 mV until the output voltages Vom and Vo1 reach the target voltage of 1.4 V.
FIG. 5C is another enlarged view of the circled part of FIG. 5A when the changing speed of the output voltage Vom is lowered in the middle of changing due to, for example, an increase of a load current of the main power source circuit. However, since the output voltage Vo1 of the sub power source circuit is increased by the speed approximately equal to the original speed, the output voltage Vo1 of the sub power source circuit becomes higher than the output voltage Vom of the main power source circuit beyond the range of ±50 mV.
In FIG. 5C, the following is not shown; however, when the output voltage Vom of the main power source circuit returns from 1.4 V to 0.7 V and the output voltage Vo1 of the sub power source circuit returns from 1.4 V to 1.1 V, a phenomenon similar to the increasing time occurs.
FIG. 5D is another enlarged view of the circled part of FIG. 5A. FIG. 5D is described below in detail.